Micturition - Anatomy & Physiology

()Map URINARY SYSTEM (Map)

Micturition is the normal process of the passive storage and active voiding of urine.

Introduction

After entering the renal pelvis the modification of the urine is over in all domestic species other than the horse where mucin is added. The urine passes along the ureters and enters the bladder. It is here that the urine is stored until it is to be voided. Urine is not constantly excreted and it is only when there is a significant amount present in the bladder that the process of voiding occurs. Both the Bladder and the Urethra have smooth muscle and thus receive autonomic influence with regard to their activity. However the urethra also has an element of skeletal muscle giving the animal some degree of conscious control over the voiding of urine.

Sensory Innervation

The bladder has stretch receptors (alpha receptors in the neck and trigone, and beta receptors in the body) which detect fullness of the bladder. These impulse to the pelvic nerves and so to the sacral spinal cord. The urethra has afferents detecting flow, distension and pain that go via the pudendal nerve to the sacral cord.

Central Intregration

The co-ordination of the urethralis and detrusor muscles from information recieved from the stretch receptors occurs in the pons. This micturition centre also branches to the thalamus, cerebellum and cerebral cortex. It is the cerebral cortex that is responsible for voluntary control of micturition.

The Muscles

The motor components of the reflex are the Detrusor Muscle, Internal Urethral Sphincter and the External Urethral Sphincter. They former two are supplied by the autonomic nervous system with the latter being of somatic innervation.

The Reflex

The Process

The bladder when empty is a small organ with folded epithelium. It is able to stretch to an organ which is much larger than when in its empty state. It has many stretch sensitive nerve endings within its wall which monitor how full it is. As the bladder is filling sympathetic supply dominates and keeps the bladder wall relaxed and the internal urethral sphincter closed. This stops urine from escaping the bladder. As the bladder fills it stretches more and pressure is placed on itself and the sphincters keeping the urethra closed. As the tone within the stretch sensitive fibres increases due to filling then we see a switch to the parasympathetic system. Above a certain level this triggers a contraction within the detrusor muscle which in turn further increases the pressure and thus further increases the tone in the sensory fibres. This is an example of a positive feedback loop. The parasympathetic tone also causes the internal sphincter to relax leaving only the external urethral sphincter to stop to urine. In untrained animals the pressure exerted on the external sphincter overcomes it by activating reflexes which over come its contraction. However in trained animals there is some level of control from the cerebral cortex and pons which reduces the autonomic reflex and maintains the contraction of the external sphincter which allows the passing of urine to be consciously delayed. This is not indefinite however and it will be overcome eventually.

Urination normally occurs until the bladder is empty thanks to positive feedback mechanisms in the urethra which increase parasympathetic tone in response to urine being present.